Field of the Invention
This invention relates to an image displaying apparatus. More specifically, this invention relates to an image displaying apparatus for displaying preview images.
Description of the Related Art
Some image forming apparatuses, for example MFPs (Multi Function Peripherals), have a preview function. The preview function is to display images (preview images) in its final form to be printed on sheets, on a display of an image forming apparatus or an operation terminal. A conventional image forming apparatus having a preview function generates a preview image in the following manner. The image forming apparatus modifies an image of a document read by an image reading unit based on a mode set during copying operation or the like, thereby the image forming apparatus generates data of printing images. The image forming apparatus compresses the printing images to a size suitable for a displaying unit with maintaining its resolution for generating the preview image.
However, according to preview images made by the above manner, the high resolution of printing images is maintained. Then, amounts of data to be processed are large when displaying preview images. In the result, it takes much time to display preview images and operability of an image forming apparatus worsened.
A technique of displaying preview images or thumbnails fast is disclosed in Document 1 below and the like. Document 1 discloses that thumbnails included in selectable pages on a display screen are generated beforehand in addition to thumbnails displayed on the display screen. Then, thumbnails can be displayed fast. Document 1 also discloses that thumbnails included in selectable pages on a display screen are generated beforehand in addition to thumbnails displayed on the display screen. Then, preview images can be displayed fast. The preview images are generated at a resolution corresponds to a maximum magnification ratio which user can select.
Further, there is another technique in which a display device generates a preview image of a next page when displaying an enlarged preview image of a page and going to the next page. The preview image of the next page has a resolution corresponds to a magnification ratio of the enlarged preview image.    Document 1: JP2006-203702 (Japan Patent No. 4446447)
According to Document 1, the resolution at which the preview images are generated beforehand corresponds to a maximum magnification ratio which user can select. The Preview images are prepared at an excessively high resolution on pages in which a user does not enlarge the preview images or a user does not enlarge the preview images at the maximum magnification ratio. Then, a lot of limitary memory spaces are excessively consumed by generating images at the excessively high resolution. It burdens a CPU (Central Processing Unit) beyond necessity and a processing speed of the CPU is slowed down. It takes more time for a displaying process of preview images and may give a user a lot of stress.
As for the aforementioned another technique, it is possible that a user decreases the magnification ratio of a preview image (reduces the size of a preview image) to select the next page after enlarging the preview image to checking the image in detail. In this situation, the displaying device generates the preview image of the next page at a resolution lower than a resolution needed for checking the image in detail. Then the displaying device has to generate the preview image of the next page again at a high resolution when the user wants to check the preview image of the next page in detail. Consequently, it takes more time for a displaying process of preview images and may give a user a lot of stress.